SECT. III.] 



CONDENSATION OF STEAM. 



113 



These quantities, derived entirely from theoretical considerations, are so near to 

 the actual effects obtained in practice, that they show us we have little to expect in 

 the form of improvement ; and with the addition of one-tenth for various causes 

 tending to decrease the effect, they may be adopted as the measure of effect in 

 those computations we have to make ; and the table affords an easy means of 

 comparing the expense of different kinds of fuel. 



191. The trials of the quantity of steam a given quantity of fuel will produce, 

 are by no means so numerous as might be expected by those who know not the 

 difficulty of ascertaining the results with precision. People shrink from the task of 

 making accurate trials, either in consequence of the great degree of attention and 

 labour they require, or the expense. The adoption of methods arising out of a 

 competent knowledge of the subject reduces both these in a considerable 

 proportion. 



The following brief collection may be useful. 



Sleek, or refuse small coal, produces about three-fourths of the effect of good 

 coal of the same species. 



We have hitherto considered effect only when fuel gives the whole, or nearly the 

 whole, of its heat ; but a certain rate of combustion and perfect management are 

 requisite to obtain this end. 



the mean temperature of the air being about 52, we have 1000 + 220 52 = 1170 nearly ; hence 

 dividing the effect in pounds of water heated 1 by 1170 we have the pounds of water that would 

 be converted into steam, and by proportion, the quantity which converts 62-5 Ibs. or a cubic 

 foot of water into steam. * 



